Advection is the transport of a substance, such as a fluid. Diffusion is the process whereby molecules move. An advection-diffusion model, then, describes physical phenomena where particles, energy, or other physical quantities are transferred inside a physical system as a result of advection and diffusion. There are computer-based advection-diffusion models that can be used to model advection-diffusion, such as the movement of salt within a river, or bubbles in a lake as they move toward the lake's surface.